A present invention is in the field of medical instruments and more particularly is directed to a device for driving a fluid-dispensing syringe.
An infusion device is generally used to control the dispensing of fluids from a syringe, for example, to a capillary tube which in turn might introduce those fluids to a patient. In many cases, it is extremely important that the rate of fluid delivery be accurately controlled.
In the prior art, several types of infusion devices have been used. One type of device incorporates a housing for supporting a syringe and utilizes an electrical motor and associated drive circuitry configured to drive the plunger of the syringe at a constant rate. Typically such configurations incorporate a lead screw coupling between the electric motor and the plunger. Alternatively, direct gear couplers have been used. While these configurations are effective for driving a syringe plunger under certain circumstances, the dynamic range of the drive motor must be large enough to accomodate changes in flow resistance encountered at the fluid output port of the syringe. Typically, such resistance variations may be accommodated by maintaining sufficient driving capability in the circuitry and motor to offset the anticipated worst case flow resistance situation. One disadvantage of this configuration is the resulting size and weight of the infusion device which provides the necessary dynamic range.
Another form of prior art infusion device is exemplified by the configuration shown in U.S. Pat. No. 4,202,333. The device shown in that patent includes a spring-driven, constant force driver for a syringe plunger. In that configuration, a syringe is placed into an operating position within a supporting housing, which includes a pivoting actuating cover. When the pivoting actuating cover is closed, with the syringe in its operating position, a pre-loaded spring is coupled to the plunger in a manner automatically applying a constant force to the plunger. With this constant force driving the plunger, the fluid output rate from the syringe is variable, depending on the flow resistance encountered at the syringe output port. One particular advantage of this configuration is that there is no need for a relatively complex plunger driving motor and associated electrical circuitry. Consequently, such infusion devices may be made relatively small and lightweight, and may also be highly reliable. However, such devices in the prior art have been limited in their versatility, since each such device is limited to provide but a single constant force to a syringe plunger. Moreover, the prior art devices have not been adapted to monitor the output flow rate, measured as a function of the plunger movement.
It is an object that the present invention to provide an improved infusion device.
Another object is to provide an improved infusion device which may selectively apply one of a number of substantially constant forces to the plunger of a syringe.
Yet another object is to provide an improved infusion device which automatically applies a selected one of a plurality of predetermined constant forces to the plunger of a syringe.
Still another object is to provide an infusion device which monitors fluid flow from a syringe in response to spring driven plunger motion in a syringe.